Circuit boards made of a ceramic have superior heat-resisting quality and durability as compared to general resin boards, and their use in, for example, personal digital assistants have been increasing. On the other hand, with a view to increase packing densities, cases in which functions as a circuit are added to ceramic boards and such boards are stacked to be used as a multilayer board have also been increasing. The green sheet is a common name for a ceramic etc. before sintering, and the board is generally subjected to processing such as perforation for forming multilayer wiring in the green sheet state.
Use of a laser beam in perforation or other processing has been increasing in view of the processing rate achieved or the facility in changing the shape of the processed hole or in view of easiness in forming a hole with a high circularity. In the following, a conventional apparatus for perforating various work pieces, especially ceramic green sheets using a laser beam will be briefly described with reference to FIG. 6.
This apparatus includes a laser oscillator 101 for generating a laser beam used for processing, a guide laser oscillation apparatus 102 for generating a guide laser beam, an optical system 120 for shaping the guide laser beam and the processing laser beam and guiding them to a predetermined position on a work piece 103, an XY stage 104 for moving the work piece 103 placed on it in the X and Y directions, a camera 105 for capturing the shape of the guide laser incident on the work piece 103 or the shape of a processed hole etc. as an image and used for positioning of the work piece, and a control system 110 for driving these components. The guide laser (for example, red light) is projected onto the work piece previously, so that correction of the position at which the laser for actual processing is projected or correction of the shape of the laser is effected based on the projection position and shape of the guide laser.
The optical system 120 is composed of total reflection mirrors 121, 123, 126, a dichroic mirror 122, a mask 124, a collimator lens 127, an XY galvano scanner mirror 128 and an fθ lens 129. The laser beam emitted from the laser oscillator 101 is deflected by the total reflection mirror 127 so as to be directed toward the dichroic mirror 122, and transmitted through the dichroic mirror 122 from its back side. Then, the laser beam is deflected again by the total reflection mirror 123 so as to be directed toward the mask 124. The guide laser beam emitted from the guide laser oscillator 102 is deflected by the dichroic mirror 122 so as to travel on the same optical path as the processing laser beam.
The processing laser beam and the guide laser beam pass through the opening 124a of the mask 124, whereby they are shaped into a form corresponding to a hole to be formed such as a approximately circular form etc. The laser beam after transmitted (passing) through the mask is a little divergent, and it is necessary to reshape it into parallel light using a collimator lens or the like. For this purpose, the laser beam after shaping is deflected by the total reflection mirror 126 so as to enter the collimator lens 127. The irradiation position of the laser beam having been made into parallel light by the collimator lens 127 is moved by the XY galvano scanner mirror 128 and the fθ lens 129 in such a way that it is delivered to a desired processing position on the work piece 103. The XY galvano scanner mirror 128 and the fθ lens 129 function together as an irradiation position control optical system for the laser beam.
The control system 110 is composed of a galvano scanner control portion 112, an image processing portion 113, a drive control portion 114 and a main control portion for controlling these portions and controlling the laser oscillator etc. in synchronization with the control by these portions. The galvano scanner control portion 112 is connected with the XY galvano scanner mirror 128 to control the irradiation position of the laser beam by controlling the XY galvano scanner mirror 128. The image processing portion 113 is connected with the camera 105. The image processing portion 113 monitors the condition, position and degree of accuracy of the processed hole based on an image obtained through the camera 105 and outputs information on the number of pulses and intensity of the laser beam to the main control portion. The drive control portion 114 drives the XY stage 104 to change the position of the work piece 103 in such a way that the position on the work piece at which a hole is to be made comes into the area that can be irradiated by the laser beam controlled by the galvano scanner mirror. This apparatus is constructed in such a way that the shape of the mask 124 is projected onto the surface of the work piece 103 at a desired reduction ratio, and a processed hole with a nearly circular shape and having little taper in its cross section is obtained.
In the above-described conventional apparatus, a large part of the laser beam is blocked by the mask 124, and only the portion that have passed through the opening 124a of the mask is used for actual processing. Accordingly, the utilization efficiency of the laser beam is not so high, and it is required to use an oscillator having a relatively large output power as the laser oscillator 101 in view of the aforementioned blocking. It is considered that the utilization efficiency of the laser affects the processing efficiency greatly especially in the case that the surface layer is made of a material having a relatively low absorption efficiency for the laser beam. In this case, the number of pulses of the laser required for processing is very large, which results in a large decrease in the processing efficiency.